1. Field
Embodiments of the present invention relate to a urea-resistant catalytic unit for reducing catalyst deactivation due to urea poisoning.
2. Background Art
While offering certain benefits in fuel economy, internal combustion engines such as diesel engines often require special exhaust aftertreatment system to reduce waste species such as nitrogen oxide (NOx) under oxidizing operating conditions.
Urea has been used as a reductant in the exhaust aftertreatment system for reducing NOx emissions in order to meet certain government and industry imposed emission regulations. For instance, aqueous urea has been injected into the exhaust stream within a selective catalytic reduction (SCR) aftertreatment system and hydrolyzed to form ammonia (NH3) which then reduces NOx over a SCR catalyst.
In vehicle applications, and as a result of space restrictions, urea residence time is often short and the liquid urea may not have sufficient time to vaporize and hydrolyze before contacting the SCR catalyst. This situation is more prevalent at lower exhaust temperature, especially below 200 degree Celsius, where liquid urea may accumulate on the SCR catalyst and cause urea byproduct formation and subsequent deactivation of the SCR catalyst.